Extra high voltage cables

ABSTRACT

For extra high voltage power cables this invention provides insulation consisting of synthetic plastic material with paper bonded to both sides to form a laminated strip. In place of the porous paper previously used, this invention uses very thin paper such as &#39;&#39;&#39;&#39;capacitor tissue&#39;&#39;&#39;&#39;; space for the thermal expansion of the synthetic and passages for the removal of moisture and the introduction of oil are obtained by embossing the strip. To prevent locking of overlying strips when bending the cable, the embossing is preferably embossed with a random pattern, or plain unembossed strips are wrapped alternately between embossed strips. The invention includes the novel strip, cable made with the strip and the method of making the strip.

I Umted States Patent 1111 3,594,439

[72] Inventors Carlos Kall 3,250,850 5/1966 Felix 174/25 X Bayonne;3,358,071 12/1967 Eich et a1. [74/25 George S. geni Upp Montclair;3,429,983 2/1969 Hofmeier 174/25 X George Bahder, Edison, all of, NJ. "P765'447 690,353 7/1964 Canada 174/25 122] FM 925 403 /1963 G tB '13174/25 Patemed July 20,1971 rea r1 1n [73] Assignee General CableCorporation Primary Examiner-Lewis H. Myers New York, N.Y. AssistantExaminer-A. T. Grimley Attorney-Sandoe, Neill, Schottler & Wikstrom [54]CABLES ABSTRACT: For extra high voltage power cables this inventionprovides insulation consisting of synthetic plastic material [52] .8. Cl174/25, with paper bonded to both sides to form a laminated trip Inl74/l 10 PM, 17 120 PP place of the porous paper previously used, thisinvention uses [51 1 Int. Cl .1 01b 7/02 very thin paper such ascapacitor tissue; space for the ther- M 0 Search 174/25, 24, ma]expansion of the synthetic and passages for the removal of 120, 120.1 1,-1, 1 1 1 l0 moisture and the introduction of oil are obtained byembossing the strip. To prevent locking of overlying strips when 1References bending the cable, the embossing is preferably embossed withUNITE STATES PATENTS a random pattern, or plain unembossed strips arewrapped al- 3,078,333 2/1963 Kang 174/25 X temately between embossedstrips. The invention includes the 3,105,872 10/1963 Thompson eta1..174/25 X novel strip, cable made with the strip and the method of mak-3,194,872 7/1965 Garner 174/25 ing the strip.

PATENTEDJUL20|97| INVENTO CARLOS KATZ 46 GEORGE s. EAGER. JR.

GEORGE BAHDER ATTORNEYS.

' EXTRA HIGH VOLTAGE CABLES I RELATED PATENTS Plastic strips havingsubstantially higher dielectric strength than paper were tried forhigh-voltage cables but the plastics were impervious to the dielectriccable oil, they did not have the stiffness to bridge helical fissuresbetween adjacent turns of the strip. The expansion of the syntheticduring a heat cycle permanently disrupted the physical integrity of thecable structure making impossible the use of conventional insulationshielding, conventional moisture and mechanical protection andconventional cable terminals. Unless the fissures could be kept open andconnected, the oil could not fill the voids. The plastic was also lessresistant than paper to the effects of corona.

The Kang U.S. Pat. No. 3,078,333 proposes the use of a dielectriccomprising a single plastic strip bonded to a single paper strip.Although the paper provides a partial mechanical reinforcement to theplastic, it does not provide protection against corona discharges in thebutt spaces or avoid the plastic swelling and/or sinking in the buttspaces. Other insufficiencies of such a cable involve difficulties insaturation and changes in diameter with changes in temperature.

The Thompson-et al. U.S. Pat. No. 3,l05,872 discloses the use ofembossed plastic tapes as dielectric. This has a number of shortcomingswhich include:

a. Unprotected plastic materials are very sensitive to coronadischarges, leading to premature cable failure under conditions causingelectrical discharges (for instance switching and lightning surges).

b. Most of the plastics are incompatible with suitable liquiddielectrics; swelling, softening, elongating or tearing under load, etc.when in contact with these liquids especially at elevated temperatures.

c. Several plastics have low stiffness sinking in the butt spaces of theunderlying tapes.

d. Most synthetic materials have poor thermal stability thus limitingthe maximum amount ofcurrent a cable can carry.

A construction which overcomes almost all of the shortcomings of otherhigh-voltage cable constructions is disclosed in Garner U.S. Pat. No.3,194,872. This Garner patent has the cable manufactured with acomposite of paper-synthetic filmpaper. The present invention is animprovement on the construction of this Garner patent.

SUMMARY OF THE INVENTION One of the improvements effected by thisinvention is a reduction in the power factor for extra high voltagecables. In such cables, the total thickness of the strip or tape must belimited to about 3 to mils. The smaller the ratio of the paper thicknessto the plastic thickness, the smaller will be the power factor. Oneproblem has been to obtain thin paper having an acceptable low airresistance which is necessary to obtain an efficient flow of themoisture out of the paper and the flow of liquid dielectric into thevoids of the paper-plastic insulation. This invention makes possible theuse of low loss, impermeable, extremely thin papers in the insulation ofcables.

Another improvement relates to providing enough space for the expansionof the synthetic used in the insulating wall, so that the overalldimensions ofthe cable are not altered and the physical integrity of thecable is not disrupted when subjected to heating, allowing the use ofthe same electrical shielding and mechanical protecting materials andthe use of the same potheads at cable terminations than in conventionalpaper-insulated cables.

Another improvement relates to the use of synthetic films having goodelectrical properties but relatively poor mechanical and thermalcharacteristics. Another improvement relates to the bonding of the paperto the plastic. In prior bonding operation, part of the plastic hasmigrated among the cellulose fibers of the paper forming barriers formoisture removal and liquid dielectric penetration.

This invention provides a composite insulation of good dielectricproperties and relatively good mechanical and thermal properties. Asynthetic film is bonded between special thin paper which occupies atotal not more than about 50 percent of the total thickness of the stripor tape. The composite strip is embossed in a pattern to provide spacefor the expansion of the synthetic and for passage of liquid dielectric.The expansion of the synthetic is produced by in service increases intemperature of the cable, One feature of the invention is an irradiationof the composite to improve the thermal stability of the dielectric.

Cable made in accordance with this invention can be wrapped with thecomposite strip or tape having only every alternate layer embossed andthe other plain. This prevents lockout of the embossed patterns when thecable is bent and does not necessitate the use of random patterns ofembossing to avoid lockout.

In addition to the novel composite strip or tape construction in thecable made with this strip or tape, the invention includes methods ofmaking the strip or tape and the cable.

The invention results in an improved cable having lower power factors,low dielectric constant, good mechanical and thermal properties, highresistance to corona discharges, and at the same time assures aneffective moisture removal and liquid saturation.

Other objects. features and advantages of the invention will appear orbe pointed out as the description proceeds.

BRIEF DESCRIPTION OF THE DRAWING I and embossed with a random pattern;

FIG. 2 is a view similar to FIG. I but showing a strip or tape embossedwith a regular pattern;

FIG. 3 is an enlarged sectional view showing a strip or tape with aregular embossing pattern wrapped between unembossed strips to preventlockup of the embossing when the cable is bent;

FIG. 4 is a fragmentary view partly broken away and in section showing acable made with the strip or tape of this invention;

FIG. 5 is an enlarged fragmentary view through several layers oftheinsulation of the tape shown in FIG. 4; and

FIG. 6 is a diagrammatic view illustrating the method of making thestrip or tape shown in the other views.

DESCRIPTION OF PREFERRED EMBODIMENTS The tape insulation shown in FIG. 1consists of a strip or tape 8 of composite material, bonded, embossedand in some cases irradiated in accordance with this invention. Thecomposite consists of a layer of suitable synthetic film I0 of the highpolymer type such as polypropylene or polyethylene respectively, bondedbetween two very thin sheets 12, each 0.00075 inch to 0.0015 inch thick,of high purity, low loss 1 paper of the type used in the manufacture ofcapacitors and known generally as capacitor tissue.

The effect of thermal expansion on synthetic materials is very large,especially when compared with the expansion of paper. For example, whenpolyethylene is heated to C. its volume increases by about 9 percent ofthe volume at room temperature, this compared with less than I percentfor conventional paper, in addition the capacitor tissue has thedisadvantage of being for all practical purposes impermeable to liquiddielectric in the radial direction. Both disadvantages are overcome bypermanently embossing the bonded composite.

The depth of embossing I4 is such that enough space is provided for theexpansion of the synthetic during the drying impregnation part of themanufacturing operation and during continuous in service operation, atthe same time the unrestricted longitudinal flow of moisture and oil isnot restricted to a relatively slow movement. At the same time it is nottoo deep to affect adversely the best dielectric performance ofthecomposite. A depth of embossing which added about 0.00]

viscosity of about 6,000 S.S.U. at 30 C.

FIG. 1 shows astrip of bonded composite embossed with a random pattern.Random embossing patterns are advantageous to avoid locking theoverlying tapesin cables where all the insulating tapes are embossed.Locking of the tapes could cause electrical and/or mechanical weaknessof thecable when it is bcnt.

FIG. Z-shows a strip 8' of bonded composite embossed with auniformlyrep'eated embossing pattern 16. The parts of the strip 8' areindicated by the same reference characters as in FIG. 1 with a primeappended. 4 A variation of the extra high voltage cable described isshown in FIG. 3. It has composite tapes 8A embossed with a random oruniformly repeated embossing pattern I8. In this variation onlyalternate layers of the dielectric need to be embossed, while theremaining layers 8B are of the same dielectric but not embossed. Paperand plastic laminations are indicated by the same reference charactersas in FIG. I with an A or B appended..Except'for the correlation ofembossed and unembossed layers, the cable made according to FIG, 3 canbe the same as that which will be described in connection with FIG. 4.

A second variation of this FIG. 3cable can be made with an embossedcomposite having capacitor tissue only at one side and synthetic film atthe other. In this case the synthetic voltage cable 22, made inaccordance with this invention and insulated with randomly embossedcomposite dielectric tapes 8. The conductor 20 of this particular cableis segmental, having two of its opposite segmentsinsulated with ahigh-purity, mechanically strong, dielectric tape'material 24. Theconductor of this cable is wrapped in an electrostatic shield 26 formedby conducting tapes over which the mass of the embossed composite tapeinsulation 8 is carefully precisely applied open butt. The direction oftape application has been changed at regular intervals (every 10 tapes)to obtain an electrically efficient and mechanically strong cable. Twolay directions of the applied insulating tapes 8 are shown. An insuaboveThe capacitor tissue 12, 12', 12A or 128 has a power factor of about0.07 percent at 80 C a dielectric constant of about l.7 at 80 C.; and anoil-impregnated dielectric strength of about 2,800 v./mil. These valuesare given by way of illustration and are more favorable than thoseof thehighest purity papers conventionally used in cable manufacture. Inaddition, because of its relatively high mechanical strength, thecapacitor tissue allows the use of synthetic film materials havingrelatively poor mechanical strength but good-electrical characteristics.

The term capacitor tissue is used herein to'designate a paper made fromshort fibered stock to obtain a pinhole free, .very high impermeablepaper, havinga thickness from 0.0002 inch to 0.00l 5 inch and a densityin the range from about 0.7 to 1.2 g./cc. The tissue has a low powerfactor and low dielectric constant.

In the preferred construction the capacitor tissue has a dry percentpower factor of about 0.07 at 80 C.,'and the power factor of the oiledcomposite of this invention is less than 0.0015 and preferably about0.0005 or less at 80 C. with an overall dielectric constant of less than3.0 at 80 C. The preferred capacitor tissue has a maximum ofconducting'particles per square foot less than L4; and has a pH of about6.3

' In the embossed composite insulated cables the capacitor tissueprovides the composite with the necessary mechanical strength(reinforcement) and protects the synthetic, when it softens and expands,from sinking in the butt spaces of the adjacent layers of dielectric.Because the capacitor tissue is strong and permanently laminated thesynthetic when a raise in temperature occurs, the synthetic cannotelongate significantly (if the paper would be weak the longitudinalexpansion of the synthetic would break the paper). The space leftbetween the tapes by the embossing is more than enough to absorb theincrease in synthetic volume.

' In addition, because the capacitor tissue protects the synthetic andprovides the composite with the necessary lation shield 30, consistingof conducting tapes, is applied over tion the removal of moisture andliquid impregnation are improved and a better high-voltage stressdistribution is obtained.

Another variation of this cable can be made by using in the cablemanufacture composite tapes having various depths of embossing. Thesedepths will depend on the thickness of the tape and'the position of thesame with respect to the conductor. with this construction, the greaterthe thickness of the tape and the further away the individual layer ofcomposite tape is from the conductor the greater is the depth ofembossing.

FIG. 5 illustrates, in detail, a longitudinal cross section'of severalembossed insulating tapes 8 ofthe cable shown inFlG. 4. The spaces leftbetween the tapes by the embossing and the butt spaces, designated bythe reference characters 34, are filled with a liquid dielectric. 1

mechanical strength it is possible to use the composite dielectricsimilarly to' paper in the manufacture of cables.

If the diameter of embossed composite insulated cables would change asisthe case with synthetic insulated cables, it would have been'necessaryto search for new materials to be used in the electrostatic shielding,moisture and mechanical protection of the cable. Only materials havingsimilar thermal characteristics to the synthetics used in these cablesand having unusual elastomeric and mechanical properties could be used.By using the embossed composite dielectric popular and well known Imaterials as, for example, copper tapes, foil backed mylar, metallicskid wires, etc. can be used. It also would have been necessary toinvent and design a kind of pothead which changes its radial dimensionswith changes in temperature.

Theembossing of the composite provides space for the thermal expansionof the plastic and permits a fast moisture removal and oil impregnationand makes negligible the possibility of void formation.

Specifically the preferred embodiment of this invention comprises anextra high voltage cable insulated with:

a A dielectric composite material consisting of a continuous sheet ofpolypropylene bonded between two thin sheets of very high purity,low-loss capacitor tissue. The bonded composite is permanently embossedwith a random or uniformly repeated pattern which may add up to about 2mils to the thickness of a single strip of composite 5 mils thick.

b. A dielectric material as above, except that the polypropylene isreplaced by polyethylene and the composite is irradiated after embossingwith a dose of 7 to 17 megarads. The irradiation of the composite willallow the efficient operation of the cable at relatively hightemperatures.

0. A dielectric material as above except that instead of polypropyleneor polyethylene other plastics are used as the central member of thecomposite.

Synthetic materials such as polyphenylene oxide, polysiloxane fluonnatedethylene propylene polycarbonate, polyimide, olysulfone.polytetrafluoroethylene, poly-4-methylpentene. polystyrene. and theirirradiated variations and all others having good electrical properties,as for example, low dissipation factor. low dielectric constant, gooddielectric strength and in addition. relatively good thennal propertiesare appropriate for use in this composite.

The extra high voltage cable of this invention is manufactured bywrapping the stranded coated or uncoatcd copper or aluminum conductorwith an electrostatic shield formed by conducting tapes.

The dielectric tape application is performed with the help ofmultiple-head taping machines, changing the direction of tapeapplication at regular intervals (for example every l0 tapes). Thisoperation is performed in a closed room, free of contaminants,maintained at about 25 to 30 C. and a relative humidity not in excess ofpercent.

After the embossed composite dielectric tapes 8 have been applied, thecable is picked up on a reel and subsequently dried under vacuum to amoisture content of less than 0.1 percent and later impregnated with aliquid dielectric which also occupies all the spaces among the soliddielectric. Shielding, moisture and mechanical protecting tapes areapplied over the composite in a subsequent operation.

The manufacture of the permanently embossed composite dielectric can beaccomplished in one continuous operation as illustrated in a simplifiedway in FIG. 6, where right after the synthetic film 10 has been extrudedit is passed between feed rolls 40 to a bonding station 44 where thefilm 10 is passed over or between a series of heated rollers 46 to whichthe very thin paper 12 is also conveyed. Previous to the meeting of thepaper 12 and the synthetic film 10, the paper 12 is passed over othersteel rollers 50 heated to about 140 C. The heating operation of thepaper serves the double purpose of removing moisture and preheating thepaper to obtain a better bond between the paper 12 and the syntheticfilm 10.

The conditions of heat and pressure, to obtain the intimate andpermanent bond, depend on the nature of the synthetic film and may be,for example, in the order of 130 to 140 C. for polypropylene and forpolyethylene. Right after the bonding has been obtained and withoutallowing the temperature to drop significantly the composite is passedbetween embossing rollers 5455, one of which is of a hard rubberlikematerial and the other is of steel having the embossing pattern engravedon its surface.

The depth and quality of the embossing will depend on the temperature ofthe composite at the time of embossing and on the speed with which thecomposite moves between the embossing rollers. For example, in case of acomposite made with a 0.0025 inch-thick film of polypropylene betweentwo sheets of capacitor tissue, each 0.001, inch, a speed of about 12feet per minute and a temperature of about 130 C. were used to obtain asuitable embossed composite having a total overall thickness of about0.0055 inch. These figures are given by way of illustration. Thepatterns used during embossing may be of any practical and feasibleconfiguration.

After the embossing operation is finished the composite is cooled to anintermediate temperature between the embossing temperature and roomtemperature, for example 60 C., at the same time that it is moved to alow relative humidity environment (to avoid, as much as possible, thepickup of moisture by the paper). The cooled composite is slit by knives62 and taken off into individual rolls 64 having width of three-fourths,seven-eighths or 1 inch, in a suitable fashion, up to, for example, inchin diameter. The rolls are placed in moistureproof containers andshipped to the side of the cable-taping machines where they will bewrapped over an electrical conductor.

During all of the above dielectric manufacturing operations special carehas to be taken to avoid any possible contamination of the material byforeign materials such as dust and moisture which could increase thedissipation factor of the composite.

The described, continuous process is the most economical for theproduction of this extra high voltage dielectric wrapping, however, thisprocess can be separated into individual operationswithout anyconsequence to the good performance of the dielectric. The embossing canalso be performed by a more complicatedprocedurein which more than oneembossing roll is used. I

in those cases where an improvement of the thermal stability of thedielectric can be obtained by irradiating the material, this operationis performed after thecomposite has been enibossed. 1n the case of acomposite made with polyethylene an irradiation dose of 7 to 17megarads, applied under vacuum is most effective. The irradiationoperation can also be performed effectively in a neutral gasenvironment.

The bond between the paper and the synthetic film can also be achievedby using any adhesive which will not adversely affect the goodelectrical and mechanical properties of the corriposite. For example,adhesives of the polyisobutylene type are suitable.

Following are some typical results concerning composite cable insulationmade in accordance with the present invention. For comparison, resultsare also shown concerning other dielectrical material.

Dissipation factors: 1

Oil impregnated, embossed composite dielectric, 5 mils thick (60%polypro- Pylene, 40% paper) O l impregnated, embossed, irradiatedcomposite dielectric, 5 mils thick (70% Polyethylene, 30% paper) 078 0limprcgnated high purity aper used in hi h voltage cables, 5 mils t ick154 ili i ir lain it di 1 tri mregnac,p compos e eec c 5 thick (60%polypropylene, 40%

aper 0B impregnated, embossed, irradiated composite dielectric, 5 milsthick (70% polyethylene, 30% paper) 2. 26 Oil impregnated high puritypaper used in high voltage cables, 5 mils t ick 3. 58 Dielectricstrength: 2

comlposite tliieletigc, 5 m)ils thick (60% o ypropy ene 0 paper C mosite irradiated dielectric 5 mils thick (7 a polyethylene 30% paper)lpurity, low density paper 5 mils c 2,800 v./mii

2,600 v./mil

1,500 v./mil

Accordlng to ASTM D2413. 2 According to ASTM D149.

Polyphenylene Oxide (al0ne)5 mus thick-broke arm 4 to 48 houi sundertest in both oils Embossed composite dielectric,

before embossing-about 6 mils after- (60% polypropylene, 30% paper).

200 hours under above conditions in both oils.

Corona Resistance Test Test performed by placing samples, 13 10.5 milsthick, under corona discharges. Keeping a clearance of 5 mils betweenthe sample and one of the electrodes and maintaining a stress of 15 kv.on the samples.

5 mils thick Negligible elongation after of a depth correlated withknown impregnation and thermal I expansion ofthe plastic and fluid toprovide space for the ex parision and to provide communicating spacesbetween overv lapping layers, of the composite insulation and their buttspaces.

'4. The electrical cable described in claim 3 characterized by l theembossing of the dielectric strips being in a random pat- Approximatehours to failure (average of several samples) Material:

. 'lomlls polypropylene plus 3 mils capacitor tissue,

polypropy ene exposed to corona discharges'. 3 mils polypropylene plus 3mils capacitor tis e, I capacitor t ssue exosed to corona dischargesCapacitor tissue alonc... 50 I High purity,.low density paper used inhigh voltage cables alone... 4. 5 '10 mils olyethylene plus .3 milscapacitor tissue, polyet ylcno expose to corona dischar es l 8 10 milspolyethylene plus" 3 mils capactor tissue.

capacitor tissue exposed to corona disehargesflu; 80

Under slightly different test conditions than above.

Oil Flow Test Tests showed that under similar test conditions the forcedflow of oil along the surface of permanently embossed composite tape isabout 10 times faster than on the surface of porous low density,highpurity paper of the type used in high.

voltage cables. l

The preferred embodiments of the invention have been illustratedanddescribed, but changes and modifications can be made, and some featurescan be used in different combinadielectric in-a radial direction, atleast some of the laminated insulation being embossed to form expansionand communicating spaces between overlapping'layers of the compositeinsulation, said spaces being filled with fluid dielectric, the paperhaving a thickness from 0.0002 inch to 0.00 l 5 inch and a density inthe range from 0.2 to L2 g./cc.

2. The electrical cable described in claim 1 characterized by thecomposite dielectric being irradiated to improve its thermal stability.

'3. The electrical cable described in claim Icharacterized by v thecomposite dielectric layers having capacitor tissue bonded to theplastic whereby the laminate including the paper is highly impermeableto liquid dielectric, the embossing being between successiveconvolutions bridged by the paper of underlying and overlying layers toprevent the plastic from flowing into the butt spaces when the cable isheated. l

5. The electric cable described in claim 3 characterized by each layerof embossed dielectric strip having a uniform pattern of embossing andbeing in alternating relation with a layer of plain unembosseddielectric strip to prevent locking of embossedsurfaces of strips withconfronting surfaces of other strips'when the'cable is bent.

6. The electrical cable described in claim I characterized by theplastic film being about 2 to 10 mils in thickness and the paper beingsheets of capacitor tissue bonded to both surfaces of the plastic.

7. The electrical cable described in claim 1 characterized by some ofthe layers of dielectric'material being of greater radial thickness thanothers, and the layers of greaterthickness have their paper sheetsembossed to a greater depth to provide more space for the expansion ofthe plastic film, and for moistureremoval and liquid impregnation.

8. The electrical cable described in claim 7 characterized by layers ofdielectric material toward the outside of the cable having plastic ofgreater radial thickness than layers nearer the center of the cable, thedepth of embossing being in accordance with the thickness of the plasticfilm of the respective layers.

9. The electrical cable described in claim 7 characterized by the layersof dielectric being irradiated.

10. The electrical cable described in claim 1 characterized byinsulation including tapes comprising plastic film bonded between sheetsof paper and including other tapes of conventional high purity, low-losspaper insulation, the conventional paper tapes being limited to not morethan about 15 percent of the total insulation cross section.

11. The electrical cable described in claim 1 characterized by theinsulation having inner and outermost layers of tape, at least oneofwhich is a paper tape.

1. An extra high voltage electrical cable including a conductor,shielding and insulation around the conductor, the insulation includingimpervious layers of laminated dielectric material comprising asynthetic plastic film of good dielectric properties and mechanicallyreinforced by a paper sheet bonded to the surface of the plastic butthinner than the plastic film said paper being highly impermeable toliquid dielectric in a radial direction, at least some of the laminatedinsulation being embossed to form expansion and communicating spacesbetween overlapping layers of the composite insulation, said spacesbeing filled with fluid dielectric, the paper having a thickness from0.0002 inch to 0.0015 inch and a density in the range from 0.2 to 1.2g./cc.
 2. The electrical cable described in claim 1 characterized by thecomposite dielectric being irradiated to improve its thermal stability.3. The electrical cable described in claim 1 characterized by thecomposite dielectric layers having capacitor tissue bonded to theplastic whereby the laminate including the paper is highly impermeableto liquid dielectric, the embossing being of a depth correlated withknown impregnation and thermal expansion of the plastic and fluid toprovide space for the expansion and to provide communicating spacesbetween overlapping layers of the composite insulation and their buttspaces.
 4. The electrical cable described in claim 3 characterized bythe embossing of the dielectric strips being in a random pattern thatdoes not lock up with the embossing of an underlying or overlying layerwhen the cable is bent, and the separate layers having helically woundlaminate with butt spaces between successive convolutions bridged by thepaper of underlying and overlying layers to prevent the plastic fromflowing into the butt spaces when the cable is heated.
 5. The electriccable described in claim 3 characterized by each layer of embosseddielectric strip having a uniform pattern of embossing and being inalternating relation with a layer of plain unembossed dielectric stripto prevent locking of embossed surfaces of strips with confrontingsurfaces of other strips when the cable is bent.
 6. The electrical cabledescribed in claim 1 characterized by the plastic film being about 2 to10 mils in thickness and the paper being sheets of capacitor tissuebonded to both surfaces of the plastic.
 7. The electrical cabledescribed in claim 1 characterized by some of the layers of dielectricmaterial being of greater radial thickness than others, and the layersof greater thickness have their paper sheets embossed to a greater depthto provide more space for the expansion of the plastic film, and formoisture removal and liquid impregnation.
 8. The electrical cabledescribed in claim 7 characterized by layers of dielectric materialtoward the outside of the cable having plastic of greater radialthickness than layers nearer the center of the cable, the depth ofembossing being in accordance with the thickness of the plastic film ofthe respective layers.
 9. The electrical cable described in claim 7characterized by tHe layers of dielectric being irradiated.
 10. Theelectrical cable described in claim 1 characterized by insulationincluding tapes comprising plastic film bonded between sheets of paperand including other tapes of conventional high purity, low-loss paperinsulation, the conventional paper tapes being limited to not more thanabout 15 percent of the total insulation cross section.
 11. Theelectrical cable described in claim 1 characterized by the insulationhaving inner and outermost layers of tape, at least one of which is apaper tape.